Hey there, first off, what type of brush do you have selected? Also, if you have settings for a tablet or stylus and you're using a mouse (for example, if the brush size is dependent on pressure), it can cause issues. Double-check your brush presets - you may have set something and not realized it. Barring that, you can always reset prefs (I think its CMD-OPT-SHIFT on startup or CTRL-ALT-SHIFT) or trash the prefs.